1. Field of the Invention
The present invention relates to an improvement of a system for encoding and decoding image information such as a sub-picture which is reproduced or played back simultaneously with a main picture.
The present invention also relates to a recording medium, such as an optical disk, and a data recording scheme for recording sub-picture data supplied together with picture data on the recording medium and simultaneously reproduced with the picture data from the recording medium.
2. Description of the Related Art
Sub-pictures superimposed on main pictures, such as titles in a movie, and image contents to be displayed as a set value of the television volume level are mainly realized by two schemes, i.e., a character code scheme and a bit map data scheme.
In the character code scheme, characters, such as pre-registered and prepared characters and patterns, are held in a character recording area of a character generator. A code assigned to each character is input to the character generator to display a desired character.
According to this scheme, dedicated hardware, such as a character generator is required, but the volume of data to be sent to a display system can be smaller than that of a scheme for directly sending character bit map data to the display system so as to display a sub-picture, because a code is input to display a character. However, the applications of display of sub-pictures according to this scheme are limited since only the pre-registered and prepared characters can be displayed.
To the contrary, in the bit map data scheme, dedicated hardware for generating a sub-picture from a code need not be used, because the bit map data of the sub-picture is directly sent to a display system. The sub-pictures can be used in a variety of display applications since the form of the sub-picture to be displayed is not fixed.
According to this method, sub-picture color data, sub-picture outline color data required for superimposing a sub-picture on a main picture, and superimposing mixing ratio data of the main picture and the sub-picture must be provided for each pixel. Therefore, the volume of data to be sent to a display system is inevitably very large. In addition, in the bit map data scheme, data associated with all pixels on a display screen (to be referred to a frame hereinafter) must be sent to the display system regardless of the size of a sub-picture (see FIG. 50). For this reason, a large amount of wasteful data are present in respect of display space.
In either the character code scheme or the bit map data scheme, even if the shape of a sub-picture being currently displayed does not change, sub-picture data must be basically kept supplied for each display frame period, and a large amount of wasteful data are present with respect to the display time.